1. Field of the Invention
The present invention relates to a device for controlling the supercharging pressure of an exhaust gas turbocharger for an internal combustion engine in which the internal combustion engine is supplied with the intake gas for supercharging by a compressor which is rotated together with an exhaust gas turbine rotated by the energy of the exhaust gas from the internal combustion engine.
2. Description of the Prior Art
It is common knowledge that the internal combustion engine is equipped with an exhaust gas turbocharger to utilize the exhaust gas energy to supercharge intake air to the internal combustion engine, thereby increasing the intake air charging efficiency and therefore the amount of fuel injection, which does greatly attribute to the increase of the engine output.
However, if the supercharging pressure becomes excessive, excessive stress is imposed on the internal combustion engine and on its intake and exhaust gas system with the attendant risk of damage or destruction. Accordingly, many devices have been developed to prevent excessive increase in supercharging pressure.
An example of these prevention devices is a velocity control device for exhaust gas flow as is described in U.S. Pat. No. 2,944,786. In this device, a control valve is provided in the exhaust gas intake port section of the exhaust gas turbocharger to adjust the degree of opening of the control valve, thereby controlling the velocity of the flow of the exhaust gas in the inlet port leading to the exhaust gas turbine so as to prevent an excessive rotation rate in the exhaust gas turbine. Consequently, the turbine is prevented from excessively driving the compressor coupled directly thereto so as not to excessively increase the supercharging pressure while increasing the exhaust gas flow velocity for higher supercharging pressure at light load operation, thereby increasing the output.
Another example is a bypass channel provided to connect the upstream and downstream sides of the exhaust gas turbine, and the bypass channel is provided with a bypass control valve means, which is adapted to open when the supercharging pressure in the bypass channel on the downstream side of the compressor goes above a set value. When the supercharging pressure on the downstream side of the compressor is about to exceed a specified value, the bypass control valve means opens and the energy of the exhaust gas is exhausted to the outside through the bypass channel so that it does not act to rotate the exhaust gas turbine. This prevents excess rotation of the exhaust gas turbine and consequently excessive increase in the supercharging pressure.
The velocity control device for exhaust gas flow and the bypass control valve device can effectively carry out supercharger pressure control, but the following types of inconveniences still remain.
Specifically, in the velocity control device for exhaust gas flow, when the supercharging pressure exceeds the set value, the cross-sectional area of the exhaust gas inlet port is enlarged to decrease the flow velocity of the exhaust gas introduced into the exhaust gas turbine. But the full volume of exhaust gas is all introduced into the exhaust gas turbine and not bypassed, so that when the exhaust gas energy is further increased, the rotational speed of the exhaust gas turbine cannot be controlled and the super-charging pressure is ultimately increased. In other words, a drawback of the velocity control valve is in insufficient flexibility of operation. Specifically, the original design specifications of the exhaust gas turbocharger must ensure that the increase of supercharging pressure is below the allowable value. For this reason, the overall torque characteristics must be set to generally produce low supercharging pressure throughout engine operation including the time that the engine is running under low load. However, this results in that the supercharging pressure is reduced in the low load region as mentioned above, which is a problem area in the case of the exhaust gas turbocharger, thus making it difficult to improve the output characteristics in this operating region.
Now turning to the bypass control valve means, when the supercharging pressure is about to exceed the set value, the exhaust gas is introduced into the bypass channel by the bypass control valve means, and is excluded from the region of rotation of the exhaust gas turbine. It therefore functions as an effective means of preventing an excessive increase in the supercharging pressure. However, the torque characteristics determined by the specifications of the exhaust gas turbocharger are constant, and simply act to cut excessive supercharging pressure, which does not solve the problem of low super charging pressure in the low load region. It should be further noted that with such a bypass control device, immediately after the bypass control valve is opened, the exhaust gas turbine rotates at a relatively excessive speed due to the effect of its inertia, causing insufficient reduction of the supercharging pressure with the production of knocking. The back pressure is precipitously lowered giving rise to the fear that the catalytic device in the exhaust system could be damaged.
In addition, it was found out that when either of these devices is used separately, if operation becomes impossible from a one time breakdown or seizure, the exhaust gas turbine and compressor rotate at an abnormally high rate and the supercharging pressure increases excessively, so that knocking could develop or engine damage could result.
Therefore, these prevention devices as mentioned above must be further improved, and development is still required in the gas intake system for supercharging in order to maintain flow flexibility and pressure variability throughout operation.